Photoelectric signal amplifier



Feb. 22, 1944.

K. THGM PHOTQELECILRIC SIGNAL AMPLIFIER Y -Filfad July 16, 1940 XXX/NVE/V TOR y K. THOM ATTORNEY Patented Feb. .22, 1944 STATES ersr OFFlCEPHOTOELEC'I'RIC SIGNAL AMPLIFIER Kurt Thtirn, Berlin I-ichtertelde,Germany; vested in the Alien Property Custodian Application July 16,1940, Serial No. 345.82%

Germany July 20, 1939 Claims.

values giving a natural and true representation of the original image orscene. It is a further object to improve the type of transmitter knownas light spot scanner which is particularly useful for the two waytransmission of images of persons or objects. A transmitter of this typecontains usually a plurality of photocells having a sensitivity versuswave length distribution corresponding to the effective light emissionof the movable spot of light which is produced, for example by a'cathode ray tube, a vapour lamp or an. incandescent lamp. If for exampleincaudescent lamps are used it is preferable to employ a photocellhaving the highest sensitivity in the red part of the spectrum. Theimage on the receiving side however makes then a chalky, flat andlifeless impression because halt-tones are not sufilciently reproduced.It has been tried to overcome this drawback by employing simultaneouslydifferent types of photocells having sensitivities in the red and in theblue part of the spectrum. This however does not produce the desiredeffect because the source of light which is used for the scanningprocess radiates either more red ormore blue light so that a goodefficiency. for both photoelectric cells cannot be obtained. A furtherdrawback or this arrangement is that the life and the primarysensitivity of the different photocells varies considerably.

It is an object of the present invention to overcome these and otherdifiiculties by means of a special amplifier arrangement in connectionwith a number of photocells having the same properties with regard tosensitivity and spectral range. According to the invention thetransmission is effected by means of a number of photocells having thesame properties and by connecting each photocell to an amplifier havingan amnlitude characteristic and a band Width which is different fromthat of the remaining amplifiers. By regulating separately the differentamplifiers the character of the image and the quality of reproductionwith respect to the gradation can be varied within wide limits andbrought to an optimal value.

Other aspects of my invention will he appareat or will be specificallypointed out in the description forming a part of this specification, butif do not limit myself to the embodiment of the invention hereindescribed, as various forms may be adopted within the scope of theclaims.

Referring to the drawing Fig. 1 shows an embodiment of the inventionwith two photoelectric cells,

Fig. 2 shows the characteristic curvesof the amplifiers arrangedaccording to Fig. 1, while Figs. 3 and t show other circuit arrangementsin accordance with the invention.

The embodiment of Fig. 1 contains two photoelectric cells I and 2connected each to a pream- 4 pllfier 3 and t. The output circuits of thepreamplifiers are connected to a further common amplifier 5. Thephotoelectric cells I and 2 are of the same type and have substantiallythe same sensitivity in the same ranges of the spectrum. In the presentcase cells using secondary emission amplification are employed. Thepreamplifiers 3 and l are different from one another on account of theiramplification factor as a function of amplitude and frequency bandwidth. Fig. 2 represents the properties of these amplifiers in a diagramshowing the amplitude a of the signal as a function of the brightness bof the original image. The light signal is amplified linearly in thephotoelectric cells l and 2. Curve 6 shows the amplification obtained bypreamplifier 3 in which the bright lights are amplified less than thesmall values of brightness. Curve 1 shows the linear curve ofpreamplifier 4 producing an undistorted amplification so that thesignals are proportional to the original brightness. The combination ofthe two amplifier outputs results in a signal producing. a distributionof brightness which appears natural and true to the eye. The use. of twoseparate amplifiers makes it possible to transmit the amplitudes ofsmall brightness values with a smaller band width than the amplitudes oflarge brightness values. This effect cannot be obtained by an amplifierhaving a characteristic correspond ing to the combined characteristic ofamplifier 3 and d. The use of amplifiers having difierent band widthsallows a considerable lowering of the shot effect for smaller values ofbrightness. This is important because in consequence of the highamplification of small amplitudes according to curve the shot noisewould be amplified to an undesired degree. The band width of thisamplifier can be made smaller because the range of smaller brightnesscontains only small differences in brightness so that no highfrequencies are present which would be necessary for a good reproductionof the image.

It is therefore a feature of the invention that photoelectric cells 6has a smaller band width than the linear preamplifier 4.

In the embodiment of Figs. 3 and 4 the two photocells I and 2 containsecondary emission electrodes and have themselves different amplityingproperties. Two different ways of achieving this object are represented.According to Fig. 3 the potential dividers II and I! of the secondaryemission photocells are so dimensioned that the currents flowing throughthe potential dividers differ from one another. The current in the arrangement ll of the photocell I is for instance. 0.5 mA. and that in thearrangement I2 of photoelectric cell the potential dividers When abright image point is transmitted corresponding to a high amplitude thevoltage of photoelectric cell I is reduced so that the amplifica- 2 is10 mA. The resistance values of 18 II and I2 are different.

tion factor also goes down. The output resistance 20 8 is made largerthan the output resistance 9 of photoelectric cell 2, in order to obtainthe same total amplitude in both circuits. The decrease of band width isobta ned by the larger anode resistance 8 of photoelectric cell I andthis has a favourable influence upon the shot noise. The potential ofthe potential divider l2 of photoelectr c cell 2 does not break down sothat the amplification factor remains constant. The photoelectric cell 2will producing an image on the receiving side having maximal definitionand sharp contrasts while the circuit of photoelectric cell I producessignals corresponding to the lower brightness values havthereforedeliver impulses 80 ing a relatively small contrast. Experiments have 85shown that in this case the eye has the impression of a correctdistribution of brightness values. The preamplifiers I4 and I5 of thisarrangement may both have a linear characteristic curve.

Fig. 4 shows a simplified arrangement. Both photoelectric cells I and 2are connected to the same potential divider I3. In order to regulate theamplification of photoelectric cell I, one or more of the leadsconnecting the grids of the secondary emission amplifier with thepotential divider I3 contain adjustable resistances ill allowing anadjustment of the potentials between single electrodes of the secondaryemission amplifier and therefore of th total characteristic of thisamplifier, so that the eifect described in connection with Fig. 3 isobtained. Alsojin this case the output resistance 8 of photoelectriccell I is made larger than the output resistance 9 of photoelectric cell2 in order to obtain the same total amplitude in both circuits.

The arrangement can be modified in various directions. The arrangementcontaining two can be employed in connection with two way televisiontransmitting booths. Each individual cell hitherto used in the booth maybe replaced by an arrangement as represented in Figs. 1, 3 or 4. It liesalso within the scope of the invention to place the photoelectric cellsI and 2 on difierent sides of the scanning 6 light ray.

The invention is not limited to the described embodiments. It can beused in connection with other types of television transmitting devices,for

example, in connection with the amplifiers of dissector tubes in orderto influence the gradation of the image.

What I claim is:

1. A television signal amplifier arrangement including a plurality ofphotoelectric cells with v a second amplifier connected to thecollecting photoelectric cells having substantially equal properties andconstruction, and a voltage divider for each photoelectric cell, saidvoltage dividers having diflerent resistance values.

-2. A television signal amplifier arrangement including a plurality ofphotoelectric cells with secondary emission multiplier electrodes, saidphotoelectric cells having substantially the same properties andconstruction, a common voltage divider for all photoelectric cells, aplurality of adjustable resistances in the leads connecting the tappingpoints of the voltage divider with a corresponding plurality ofsecondary emission elec trodes of one of said cells while thecorresponding electrodes of the other of said cells are connecteddirectly to said tapp points.

3. In a television system, a photoelectric electron multiplier having aplurality of multiplying stages, a second photoelectric electronmultiplier substantially like said first multiplier, a voltage dividercomprising a resistance having voltage taps corresponding to saidmultiplying stages, means to connect the taps of said voltage divider tothe stages of said first multiplier, means to .connect the same taps ofsaid voltage divider to the corresponding stages of said secondmultiplier comprising a variable resistance connected in at least one ofsaid connections, and a common circuit associated with said multipliersforthe current resulting from concurrently energizing both saidmultipliers from the same succession of elemental areas.

4. In a television system, a photoelectric electron multiplier having aplurality of multiplying electrodes and a collecting electrode, a secondphotoelectric electron multiplier substantially identical with saidfirst-mentioned multiplier, an amplifier connected to the collectingelectrode of said first multiplier, said amplifier being adapted toamplify currents corresponding to large light values on said firstmultiplier to a lesser degree than currents corresponding to small lightvalues,

electrode of said second multiplier, said second amplifier being adaptedto amplify currents corresponding to both large and small light valueslinearly, and a third amplifierconnected to both said first and secondamplifiers for concurrently amplifying currents received from both saidamplifiers.

5. In a television system, a photoelectric electron multiplier having aplurality of multiplying electrodes and 9. collecting electrode, asecond photoelectric electron multiplier substantially identical withsaid first-mentioned multiplier, an amplifier connected to thecollecting electrode of said first multiplier, said amplifier beingadapted to amplify currents corresponding to large light values on saidfirst multiplier to a lesser degree than currents corresponding to smalllight values, a second amplifier connected to the collecting electrodeof said second multiplier, said second amplifier being adapted toamplify currents cor responding to both large and small light values 0linearly, said first amplifier being designed to transmit a narrowerband of frequencies than said second amplifier, and a third amplifierconnected to both said first and second amplifiers for concurrentlyamplifying currents received from both said amplifiers.

6. In a television system, a photoelectric electron multiplier having aplurality of multiplying electrodes, a voltage divider for saidmultiplier having voltage taps connected to said multiplysecondary"emission multiplier electrode aid ing electrodes, the current in saidvoltage diof the order of mA., means to impress the same voltageon bothsaid voltage divideraand means to impress the signaling current fromboth said multipliers on a common circuit.

7. In a television system, a photoelectric electron multiplier having aplurality of multiplying electrodes, a voltage divider for saidmultiplier having voltage tam connected to said multiplying electrodes,the current in said voltage divider being of the order of 0.5 mA., a,second photoelectric electron multiplier having the same number ofmultiplying stages as said first multiplier, a voltage divider for saidsecond multiplier having corresponding voltage taps connected tocorresponding multiplying electrodes, the current in said second voltagedivider being of the order of 10 mA., means to impress the same voltageon both said voltage dividers. linear amplifiers indi vidual to saidmultipliers connected to said multipliers to amplify signaling currenttherefrom, and means to impress the signaling current from saidamplifiers on a common circuit.

8. In a television system, a photoelectric electron multiplier having aplurality of multiplying electrodes, a voltage divider for saidmultiplier having voltage taps connected to said multiplying electrodes,the current in said voltage divider being of the order of 0.5 mA., asecond photoelectric electron multiplier having the same number oimultiplying stages as said first multiplier, a voltage divider for saidsecond multiplier having corresponding voltage taps connected to cor--responding multiplying electrodes, the current in said second voltagedivider-being of the order of 10 mA., means to impress the same voltageon both said voltage dividers, a linear amplifier having a large inputcoupling resistance connected to the output of said first multiplier, asecond linear amplifier having a smaller input coupling resistanceconnected to the output of said second multiplier, said couplingresistances being so proportioned that the maximumamplitudes oisignaling currents in both circuits is the same and the band width ofsignaling current is less in said first amplifier than in said secondamplifier, and means to impress the signaling currents from both saidamplifiers on a common circuit.

9. In a television system, a first photoelectric electron multiplierhaving a photoelectric cathode, a plurality of secondary electronemissive electrodes, a collecting electrode and a retarding electrode, asecond photoelectric electron multiplier substantially identical withsaid first multiplier, a voltage divider having voltage taps supplyingprogressively increasing voltages from negative to positive, connectionsfrom the most negative tap to both said photoelectric cathodes,connections of negligible resistance from succeeding positive taps tocorresponding secondary electron emissive electrodes of bothmultipliers, a connection including a large resistance from the mostpositive tap to said collecting electrode of said first multiplier, aconnection including a smaller resistance from said most positive tap tosaid collecting electrode of said second multiplier, V a connectionincluding a variable resistance from the next to the most positive tapto the retarding electrode of said first multiplier, a connection ofnegligible resistance fromsaid same nextto the most positive tap to theretarding electrode of said second multiplier, a variable resistanceincluded in the connection to the secondary electron emissive electrodeof said first multiplier which is adjacent to said collecting electrode,linear amplifiers coupled to said large and small resistancesrespectively, and means to impress signaling current from both saidamplifiers on a common circuit;

10. In a television system, a first circuit comprising a photoelectricelectron multiplier having a plurality of multiplying electrodes and acollecting electrode and an amplifier connected to the collectingelectrode of said multiplier, said circuit being adapted to amplifycurrents corresponding to large light values on said multiplier to alesserdegree than currents corresponding to small light values, a secondcircuit comprising a second photoelectric electron multiplier alsohaving a plurality of multiplying electrodes and a collecting electrodeand a second amplifier connected to the collecting electrode of saidsecond multiplier, said second circuit being adapted to amplify currentscorresponding to both large and small light values linearly, said firstamplifier being designed to transmit a narrower band of frequencies thansaid second amplifier, and a common circuit additively associated withboth said first and second amplifiers to receive current resulting fromconcurrently energizing both said multipliers from the same successionoi elemental areas.

KURT mom.

